Droplet deposition apparatus

ABSTRACT

An ink supply system for a droplet deposition apparatus wherein the pressure at the nozzle is controlled by a remote point, said remote point being positioned in parallel with said print head. The flow restrictions in the printhead arm and the pressure control arm of the circuit being selected to achieve this.

The present invention relates to printers and in particular dropletdeposition ink jet printers

Ink jet printers are no longer viewed simply as office printers, theirversatility means that they are now used in digital presses and otherindustrial markets. It is not uncommon for print heads to contain inexcess of 500 nozzles and it is anticipated that “page wide” print headscontaining over 2000 nozzles will be commercially available in the nearfuture.

These print heads are typically “end shooters” i.e. the channel orejection chamber has an ink inlet and a nozzle through which the ink isejected. Ink flows into the chamber via the ink inlet and the only wayfor the ink to leave the chamber is via the nozzle.

It has been found that certain benefits are achieved where an ink outletis added to the ejection channel in addition to the ink inlet and theejection nozzle. Ink is caused to flow through the channel—even whileprinting—which helps to reduce the probability of particles or bubblesblocking the nozzle.

Because of the size of these industrial printers, a large amount of inkis ejected from the heads when printing full black i.e. all the ejectionchambers are printing at their maximum rate. It is proposed in printheads of the prior art that a flow rate through the print head of aroundten times the maximum printing rate is used in order to help flush dirtout of the print head and maintain the head at a constant temperature.

It is preferred that the nozzles are kept at just below atmosphericpressure since a pressure above atmospheric may result in weeping ofejection fluid and pressures significantly below atmospheric may causethe sucking of air into the ejection chamber. Neither of these effectsprovide stable operation and are therefore undesirable.

Because of the ink circulation, there is provided an inlet manifold andan outlet manifold. There is a significant pressure drop in the printhead between the inlet and outlet manifolds and to ensure the correctpressure at the nozzle the pressures in both the inlet and outletmanifolds may be specified. The inlet manifold pressure being positiveand the outlet manifold pressure being negative and of a slightlygreater magnitude than the inlet pressure.

These pressures can be achieved using a gravity feed system utilising anupper and lower reservoirs, ink supplied to the print head from theupper reservoir and a pump being provided to return the un-ejected inkthat collects in the lower reservoir back to the upper reservoir. Inorder to provide the necessary pressures.

Whilst this arrangement is acceptable for static applications and wherea large machine is not an issue, there is a need for an ink supplysystem that is more compact. It is an object of the present invention toaddress this and other problems.

Accordingly, the present inventions consists in one aspect in dropletdeposition apparatus comprising: at least one print head each having atleast one nozzle for ejecting fluid from that print head; fluid supplymeans for supplying fluid under pressure to said at least one printhead; and pressure control means, located in said fluid supply means inparallel with the or each print head, for adjusting fluid pressurewithin said fluid supply means in order to control the fluid pressure atthe or each nozzle.

Preferably, pressurising means are located in said fluid supply means inparallel with the or each print head and said pressure control means.

Advantageously, a junction is provided in said fluid supply meansdownstream of said pressurising means wherein, said junction dividessaid fluid supply means into at least two arms, and where downstream ofsaid junction said pressure control means are located in one arm and theor each print heads are located in a different arm.

Suitably, a further junction is provided in said fluid supply meansdownstream of said pressure control means and wherein, said furtherjunction combines fluid in the arm from said pressure control means andfluid in the arm from the or each print head into a combined conduit.

According to a preferred embodiment a junction is provided downstream ofthe pump, and fluid is directed along one arm to the print head andalong the other arm to a pressure reference point, said arms combiningat a further point to form a single conduit that feeds the pump.Reference point A is connected to means capable of adjusting thepressure at reference point A and consequently the pressure at thenozzle. In the preferred embodiment this is a small reservoir open tothe atmosphere and which can be raised or lowered in order to affect thepressure at the nozzle. In alternative embodiments the means foradjusting the pressure is a pressurised container.

Careful selection of the resistances in the pressure reference arm withreference to the resistance in the printhead allows for control of thepressure at the nozzle by manipulating the pressure at a remote pointarranged in parallel to the printhead.

Preferably, the flow resistance upstream of reference point A andupstream of the at least one nozzle are substantially identical and theflow resistance downstream of reference point C and downstream of the atleast one nozzle are also substantially identical. The flow resistanceof the upstream and downstream conduits either side of reference point Abeing substantially the same.

The flow resistance in the conduit either side of reference point A canbe specified through the use of restrictors. The restrictors can besimple hardware, such as pipes having a particular flow resistance, ormore complex hardware such as valves and the like. If pipes are used itis preferable that substantial lengths of a moderate inner diameter areused rather than short lengths of a narrow inner diameter; erosion andaccretion of dirt will then be unlikely to spoil the symmetry of thesystem.

Ink preferably flows at a higher rate round the pressure control armthan round the print head arm of the circuit which means that a dirtparticle within the circuit less of a chance to flow through the printhead simply because more ink flows through the pressure control arm.

The symmetry of the system is not perfect however, as the pump and thefilter cannot both be placed on the “plane of symmetry”. However, pumpdegradation and filter loading, within reason, do not significantlyaffect matters. Even substantial pressure drop through the filter, orpump wear merely lowers the flowrate through the main restrictors andhence the pressure drop across them. This in turn reduces the flow ratethrough the print head, which is not critical.

Another element of asymmetry is the fact that ink is ejected from theprint head, so while a particular flow enters the head, a smaller amountremains in the conduit down stream of the print head. Typically a flowof 10 times maximum printing rate enters the head and correspondingly aflow of between 9 and 10 times maximum printing rate leaves the head. Anamount of ejection fluid between 0 and 1 times the maximum printing ratebeing ejected by the print head.

Ink to make up amount of ink that is ejected by the print head ispreferably added to the supply circuit at the point at which the twosupply arms combine downstream of the nozzles and the pressure referencepoint A.

In a further embodiment of the present invention, the print head ismounted onto a scanning carriage. The bulk supply reservoir and thepressure adjustment reservoir are mounted onto a static part of theprinter, all the other equipment mounted onto the carriage.Accelerations at the ends of the carriage motion are controlled bybuffering the resulting pressure fluctuations at A. Alternatively, thepressure adjustment reservoir can be mounted on the scanning carriage ata point below that of the nozzles in the print head. Beneficially thisreduces the effects of the acceleration on the pressure within thesupply circuit.

In another aspect, the present invention consists in a method ofproviding a flow of ink through an ink chamber having an ink inlet portat which a positive ink pressure is established, an ink ejection orificeand an ink outlet port at which a negative ink pressure is established,characterised by the flow of ink external to the chamber through aseries connection of a first flow restrictor, a reference pressuredevice and a second flow restrictor to define respective positive andnegative ink pressures at the ends of the first and second flowrestrictors remote from the reference pressure device and theapplication of said positive and negative ink pressures to the inlet andoutlet ports respectively of the ink chamber.

Advantageously, the reference pressure device operates through exposureof an ink surface to a defined air pressure which is preferablycontrollable and may be atmospheric pressure.

Suitably, the first and second flow restrictors are balanced with therestriction to ink flow in the chamber between the ink inlet port andthe ink ejection orifice and between the ink ejection orifice and theink outlet port so that the ink pressure at the ink ejection orifice isdefined by the reference pressure device.

In yet another aspect, the present invention consists in a method ofsupplying ink to a print head where the pressure at the nozzle iscontrolled by a remote point, said remote point being positioned inparallel with said print head.

In still a further aspect, the present invention consists in a method ofsupplying ink to an ink chamber having a nozzle, wherein parallel flowsare established in the ink chamber and in a pressure control path; theparallel flows being balanced such that the pressure at the nozzle isdefined by the pressure applied at a reference point in the pressurecontrol path.

Advantageously, the pressure control path comprises a series connectionof a first flow restrictor, a reference pressure device defining saidreference point and a second flow restrictor.

Preferably, the reference pressure device operates through exposure ofan ink surface to a defined and preferably controllable air pressure,which may be atmospheric pressure.

Suitably, the flow of ink through said pressure control path is greaterthan the flow of ink through the ink chamber.

The present invention will now be described, by way of example only,with reference to the following drawings, in which:

FIG. 1 is a gravity feed ink supply circuit according to the prior art;

FIG. 2 depicts a through flow ink jet print head;

FIG. 3 is an expanded view of the print head of FIG. 2;

FIG. 4 depicts an ink supply circuit for a single row print headaccording to the present invention;

FIG. 5 depicts an ink supply circuit for a double row print headaccording to the present invention;

FIG. 6 depicts an ink supply circuit for a page wide array; and

FIG. 7 depicts a further circuit for a print head.

FIG. 1 depicts a gravity feed ink supply system according to the priorart. A print head 1 is capable of firing a liquid 2 from nozzles locatedon the underside of the head. The ink chambers that eject the nozzlesare arranged in two parallel arrays and supplied with ink from a centralmanifold 3 and un-ejected ink is removed from the print head by twooutlet manifolds 4.

Ink is continually supplied to the print head from an upper reservoir 5,the level of liquid within the reservoir being controlled by a levelsensor 6. The rate of ink flow is of the order ten times the maximumdrop ejection rate. Because of the small size of the ejection chambersand the high pressure drop across them, a high pressure is requiredgoing into the print head in order to realise a slightly negativepressure at the nozzles. This pressure is achieved through the provisionof a pressure head H_(u) which is the difference between the height ofliquid in the reservoir and the nozzles. Typically the pressure at theinlet manifold must be of the order +2800 Pa.

The nozzles in the chambers are located mid way between the inletmanifold 3 and the outlet manifold 4. The pressure drop in the printereither side of the nozzle is therefore substantially identical. Ink thatis flowing through the chamber passes to a lower reservoir, in which thelevel of liquid is controlled by a level sensor 8. The height differenceH_(L) between the nozzles and the surface of the fluid in the lowerreservoir defines the pressure at the nozzles, which must be at asubstantial negative pressure of approximately −3200 Pa. This achieves apressure at the nozzle that is just below atmospheric.

Ink is returned to the upper reservoir via a filter 10 using a pump 9.In this arrangement, the print head and pressure reference points arearranged in series.

Typically H_(U) is of the order 280 mm and H_(L) of the order 320 mm. WO00/38928 (incorporated herein) describes this ink supply in greaterdetail and consequently it will not be described in any more detailhere.

FIG. 2 is a perspective view of a continuous flow drop on demand ink-jetprint head. A block of piezoelectric material 24 has channels 32 formedby a sawing process. The piezoelectric block is polarised in itsthickness direction and electrodes (not shown) are provided on eitherside of each wall bounding the channels. Upon activation of a fieldbetween the electrodes on opposing sides of the walls, the walls deflectin shear and hence pressurise the ink contained within the channels.This causes a drop to be ejected from the nozzles 30 formed in a coverplate 34. The mechanics of such drop ejection is well known anddescribed in the prior art, see for example EP-A-0 277 703 or EP-A-0 278590 and incorporated herein.

This structure and other structures, single and double row actuators arealso well known in the prior art; see WO 00/24584 and WO 00/29217amongst others (both of these applications incorporated herein).

In this single row actuator, ink is supplied to the actuator throughports 20 formed in a base 26 and removed from the actuator through ports22 also located in the base, but at the opposite end of the channel. Asupport 28, with the cover 34 and the base 26 defines a manifold.

The present invention will now be described with reference to FIGS. 3 to6.

FIG. 3 is an expanded view of the print head of FIG. 2. The nozzles 30are located midway along the channels 32. The dimensions of each of thechannels are relatively small; typically the width is of the order 75microns, the depth of 300 microns and the length approximately 1 mm.Since the head is capable of printing drops up to 50 pl at a frequencyaround 6.2 kHz, the greatest flow rate through the nozzles is about3.1×10⁻¹⁰ m3/s and thus at 10 times this flow rate, the velocity alongthe channel is 0.14 m/s.

Because some of the ink is ejected from the nozzles, the pressure dropalong the first half of the channel is greater than that along thesecond half of the channel. In theory, these can be shown schematicallyas two restrictors, 56 and 58 in FIG. 4.

The ink supply according to the preferred embodiment of the presentinvention is depicted in FIG. 4. A single row, through flow print headis positioned in parallel with a pressure reference point A. Referencepoint A and the nozzles 30 are in a fixed spatial relationship with oneanother and with a pump 52 positioned so as to be able to supply ink toboth reference point A and the nozzles simultaneously.

Unejected ink that flows from the print head is combined with inkflowing from the reference point A and used to feed the pump. Ink toreplace that which is ejected from the nozzles is supplied to the inkdownstream of either or both the reference point A and the nozzles froma bulk supply reservoir 54.

Schematically, the channels and manifolds within the print head aredepicted as restrictors 56 and 58. Because the nozzles are positionedcentrally within the channels, each of the restrictors 56, 58 providesubstantially the same resistance.

Located either side of the reference point A are restrictors 60,62.These are balanced with one another so that when ink is flowing roundthe circuit a positive pressure of approximately +2800 Pa is establishedat the opposite side of restrictor 60 and a negative pressure ofapproximately −3200 Pa is established at the opposite side of restrictor62.

The circuits are balanced so that the pressure entering the printhead(i.e. upstream of restrictor 58) is similarly +2800 Pa and the pressureleaving the printhead (i.e. downstream of restrictor 56) is similarly−3200 Pa. Because of the pressure drops provided by the restrictors,this establishes a pressure at the nozzles 30 that is substantially thesame as that at the pressure reference point A.

The restrictors can simply be a length of pipe, either a short piecewith a narrow bore or a longer piece with a larger bore. In thisexample, the bore is of a moderate inner diameter so that erosion orbuild up of dirt will not have a significant effect on the symmetry ofthe system. Alternatively, the use of a valve will provide a greateroperating freedom.

The pressure at the reference point A is controlled by the height of theliquid contained within the small control reservoir 64 with is open tothe atmosphere. By raising the reservoir higher, the pressure atreference point A is increased and subsequently all the pressures withinthe supply circuit are also increased by a corresponding amount. By thissimple movement, the pressure at the nozzles can be raised.

Similarly, by lowering the control reservoir, the pressure at referencepoint A is decreased and subsequently all the pressures within thesupply circuit are also decreased by a corresponding amount. By thissimple movement, the pressure at the nozzles can be lowered.

By altering the pressure within the small reservoir, it is possible toeffect purging or sucking at the nozzles for maintenance purposes.

Turning to the hydraulic flows within the supply circuit, the pump mustbe sized so as to be able to achieve a flow of at least 10 times themaximum ejection rate through the print head and a flow, preferably inexcess of this, through the pressure reference point A. A higher flowthrough the pressure reference point A of around 20 times the maximumejection rate being preferred.

The pump must therefore be capable of pumping 30 times the maximumejection rate i.e. 9.3×10⁻9 m³/s. Make up ink is supplied to the systemat a rate of between 0 and 3.1×10⁻¹⁰ m³/s. Whilst this is typically notsupplied in a smooth flow, because it is joining a flow around 30 timeslarger any pressure fluctuations are negligible. Indeed, it has beenfound that the system is tolerant to any flow surges caused by the pump.It is believed that the reason for this is that as the pump is locatedas a component in the circuit a fluctuation in flow at the pump outletis matched by a corresponding fluctuation in flow at the pump inlet.

As the flow rate past the pressure reference point A is twice that ofthe flow through the head, any dirt particle in the system which avoidsbeing caught in the filter 66 has twice the chance of flowing round thepressure reference circuit than through the print head. As the particlemust pass through the filter 66 a second time before having a secondopportunity to flow through the print head. Thus, the chance of any oneparticle causing a blockage in the print head is further reduced.

Whilst a higher flow rate of ink past the pressure reference point A isdesirable it is no means essential. The important rate of flow is thatthrough the print head and since this flow volume is preferably tentimes the maximum printing volume there are, at least, nine times themaximum printing volume leaving the print head outlet. The probabilityof blockages is therefore reduced without a large flow passing throughthe pressure reference point.

The schematic for a double row print head is depicted in FIG. 5. The inkis supplied to both rows from a single central manifold in parallel andnon-ejected ink from both rows of ejection chambers is combined at anexit manifold.

The dashed line B-B in FIGS. 4 and 5 denotes the placement of equipmentin a scanning application according to a further embodiment of thepresent invention. The circuit to the right of the line is placed ontothe scanning carriage, whilst the reservoirs to the left of the line B-Bare fixed.

Pressure fluctuations caused by acceleration of the carriage may bebuffered using the small reservoir 64. As the pipe between the smallreservoir and the pressure reference point A may be smaller that thepipes carrying the flow of ink around the circuit the pressurefluctuations may be controlled by relatively small changes in altitudeof the small reservoir or, where the small reservoir is closed toatmosphere and the pressure actively controlled, relatively smallchanges to the pressure in the air space above the liquid.

In an alternative embodiment for the scanning arrangement the smallreservoir may be mounted on the carriage. Where this is positioned belowthe print head no static pressure reference reservoir is required. If,however, it is inconvenient to place the small reservoir below the printhead, it may be placed above and an air pipe running from the smallreservoir to a static pressure control device may be used to establishthe correct pressure at point A. Beneficially the air pipe does not giverise to a pressure difference under acceleration.

FIG. 6 depicts an ink supply for a page wide array. A main pump 100circulates ink around a circuit that contains both a pressure controlreservoir 102 and a print head 104.

Downstream of the pump is a flow control valve 106 and a filter 108 forremoving dirt particles. The flow control valve maintains a steady flowof between 1 and 7 litres per minute. The bore of the pipe is around 10mm in diameter.

Downstream of the filter, the circuit splits into two separate circuitsin parallel. The first, marked, 110,112,114 is formed of a narrow boretubing and includes a connection to a pressure control reservoir 102open to atmospheric pressure. The narrow bore tubing is of the order 2mm in diameter and its length is such that the pressure in the pressurecontrol reservoir is reflected at the nozzles of the print head. Thepressure control reservoir 102 contains around 100 ml of ink.

The second circuit 110, 116, 114 contains the print head 104. A by-passvalve 118, which is usually closed, and flow meter 120 are provided tofacilitate operation. The flow through the head is typically between 1and 7 litres per minute. The bore of the pipe is of the order 10 mm.

The two circuits combine at point 114 and the ink is circulated back tothe pump 100. Ink from a make-up circuit is added at this point. Themake up circuit has a pump 122 providing a flow below 1 litre perminute. The ink is filtered and supplies the pressure control reservoir102. The make up ink for supply to the main pump 100 is removed at thispoint.

The level of ink in the pressure control reservoir is controlled by aweir, excess ink flowing out of an outlet to a lower bulk ink reservoir124 used to supply the make-up pump 122 A more elegant ink supply can beachieved by supplying the main filter 108, the pressure controlreservoir 102 and the narrow bore tubing 130,132 as a single unit asshown in FIG. 7.

In this embodiment the pressure control reservoir 102 is placed in thesingle unit in a position above the filter and the unit itself has asize of the order 10 cm×10 cm×20 cm. For ease of reference the portionof the single unit comprising the pressure control reservoir is calledthe header portion and the portion comprising the filter the filterportion. The header portion is 3 cm in height and a weir 134 determinesthe level of liquid in the header portion which is open to atmosphere. Asmall bleed hole 136 allows air to pass from the filter portion to theheader portion.

Top-up fluid to replace that printed by the print head 104 is suppliedfrom a reservoir via a pump 122. The top-up fluid is supplied directlyto the header portion and any excess flows over the weir 134 and returnsto the reservoir via a non-porous tube 138 in the filter portion. Thetop-up fluid may be filtered prior to entering the header portion. Theflow of ink through this portion is relatively low and typically wellbelow 1 litre/minute.

Turning to the main ink circulation circuit, a pump, preferably a magnetpump supplies the fluid to a cooler to cool the ink before it reachesthe filter portion. The outlet of this tube is located in the hollow ofa filter. The filter 108 is preferably a tubular filter with a 5 cm ODand a height of 13 cm and a pore size of 5 μm. The ink flows through thefilter and an outlet positioned towards the base of the filter housingis used to take ink to the print head. Beneficially this structure makesthe system tolerant to air as any air must pass through the filter,rather than the bleed portion 136 and then downwards through the ink inthe filter housing before passing to the print head.

The narrow bores 130 and 132 allow a flow of ink from the print headinlet to the print head outlet via the header portion and act as twoarms of a bridge. The level of the fluid in the header tank portion isthe pressure reference and sets the pressure at the nozzles.

Ink flows at a reasonable velocity through the narrow bore tubes and thepressure control reservoir 102 should be of a size such that no air issucked down the return bore 132.

The resistances of these tubes are matched to the inlet and outlet tubesto the print head and the flow of fluid to the print head is of theorder 1 litre/minute. The size of the tubes supplying ink to and fromthe print head must be of a size that allows for a sufficient velocityof ink to prevent air collecting; yet large enough to prevent anexcessive pressure drop. It practice it has been found that a 10 mm borewith an inside diameter of 7 mm will works well. Where the diameter is12 mm with an inside diameter of 10 mm is used it has been found theflow of ink is low enough to allow some air to collect however this aircan easily be dislodged back into the ink stream by gentle tapping.

Each feature disclosed in this specification (which term includes theclaims) and/or shown in the drawings may be incorporated in theinvention independent of or in combination with other disclosed and/orillustrated features.

1. A droplet deposition apparatus comprising: at least one print headeach having at least one nozzle for ejecting fluid from that print head;fluid supply means for supplying fluid under pressure to said at leastone print head; and pressure control means, located in said fluid supplymeans in parallel with the or each print head, for adjusting fluidpressure within said fluid supply means in order to control the fluidpressure at the or each nozzle.
 2. Apparatus according to claim 1,wherein pressurizing means are located in said fluid supply means inparallel with the or each print head and said pressure control means. 3.Apparatus according to claim 2, wherein a junction is provided in saidfluid supply means downstream of said pressurizing means, wherein saidjunction divides said fluid supply means into at least two arms, andwhere downstream of said junction said pressure control means arelocated in one arm and the or each print heads are located in adifferent arm.
 4. Apparatus according to claim 3, wherein a furtherjunction is provided in said fluid supply means downstream of saidpressure control means and wherein said further junction combines fluidin the arm from said pressure control means and fluid in the arm fromthe or each print head into a combined conduit.
 5. Apparatus accordingto claim 4, wherein said combined circuit supplies said pressurizingmeans with fluid.
 6. Apparatus according to claim 2, wherein saidpressurizing means is a pump.
 7. Apparatus according to claim 3, whereinthe resistance of the arm between said junction and said pressurecontrol means and said junction and said nozzle in the or each printhead is substantially the same.
 8. Apparatus according to claim 3,wherein the resistance of the arm between said pressure control meanssaid further junction and said nozzle in the or each print head and saidfurther junction is substantially the same.
 9. Apparatus according toclaim 1, wherein said pressure control means is a reservoir containing afluid having a surface open to atmospheric pressure.
 10. Apparatusaccording to claim 9, wherein means are provided that can raise or lowersaid reservoir.
 11. Apparatus according to claim 9, wherein said surfaceis at a lower altitude that said nozzles.
 12. Apparatus according toclaim 9, wherein said surface is at a higher altitude that said nozzles.13. Apparatus according to claim 1, wherein said nozzle is located in anejection chamber.
 14. Apparatus according to claim 13, wherein saidejection chamber is supplied with fluid from an inlet manifold and inkis removed from said ejection chamber by an outlet manifold, said inletand said outlet manifolds being different manifolds.
 15. A method ofproviding a flow of ink through an ink chamber having an ink inlet port,an ink ejection orifice, and an ink outlet port comprising establishinga positive ink pressure at the ink inlet port, establishing a negativeink pressure at the outlet port, and flowing ink external to the chamberthrough a series connection of a first flow restrictor, a referencepressure device, and a second flow restrictor to define respectivepositive and negative ink pressures at the ends of the first and secondflow restrictors remote from the reference pressure device, and applyingsaid positive and negative ink pressures to the inlet and outlet ports,respectively, of the ink chamber.
 16. A method according to claim 15,comprising operating the reference pressure device through exposure ofan ink surface to a defined air pressure.
 17. A method according toclaim 16, wherein the defined air pressure is controllable.
 18. A methodaccording to claim 17, wherein the defined air pressure is atmosphericpressure.
 19. A method according to claim 18, wherein the height of saidink surface is controllable.
 20. A method according to claim 15,comprising balancing the first and second flow restrictors with therestriction to ink flow in the chamber between the ink inlet port andthe ink ejection orifice and between the ink ejection orifice and theink outlet port so that the ink pressure at the ink ejection orifice isdefined by the reference pressure device.
 21. A method according toclaim 15, wherein the flow of ink through said series connection isgreater than the flow of ink through the ink chamber.
 22. A methodaccording to claim 15, comprising applying the respective positive andnegative ink pressures to a common ink inlet port and a common inkoutlet port of a plurality of ink chambers connected in parallel. 23.Method of supplying ink to a print head comprising controlling thepressure at the nozzle by a remote point, said remote point beingpositioned in parallel with said print head.
 24. A method of supplyingink to an ink chamber having a nozzle, comprising establishing parallelflows in the ink chamber and in a pressure control path; and balancingthe parallel flows such that the pressure at the nozzle is defined bythe pressure applied at a reference point in the pressure control path.25. A method according to claim 24, wherein the pressure control pathcomprises a series connection of a first flow restrictor, a referencepressure device defining said reference points and a second flowrestrictor.
 26. A method according to claim 25, comprising operating thereference pressure device through exposure of an ink surface to adefined air pressure.
 27. A method according to claim 26, wherein thedefined air pressure is controllable.
 28. A method according to claim26, wherein the defined air pressure is atmospheric pressure.
 29. Amethod according to claim 28, wherein the height of said ink surface iscontrollable.
 30. A method according to claim 15, wherein the flow ofink through said pressure control path is greater than the flow of inkthrough the ink chamber.
 31. A droplet deposition apparatus comprising:a first fluid circuit arm containing at least one print head having atleast one nozzle for ejecting fluid from that print head; a second fluidcircuit connected in parallel with said first fluid circuit arm andcontaining a fluid pump; and a third fluid circuit arm connected inparallel with said first and second fluid circuit arms and containing apressure control device; wherein the flow resistance in the first fluidcircuit arm upstream of the nozzle is substantially equal to the flowresistance in the third fluid circuit arm upstream of the pressurecontrol device, and wherein the flow resistance in the first fluidcircuit arm downstream stream of the nozzle is substantially equal tothe flow resistance in the third fluid circuit arm downstream of thepressure control device.
 32. Apparatus according to claim 31, whereinsaid pressure control device is a reservoir containing a fluid having asurface open to atmospheric pressure.
 33. Apparatus according to claim32, wherein said reservoir is adjustable in height.